Color cathode ray tube with reduced halo

Abstract: An electron beam aperture (1a) of a second grid electrode (2, 11) of a color cathode ray tube is a rectangular hole and formed in the center of a slit-like recess (1b) having longer sides extending in the horizontal direction. The vertical width (d) of the rectangular hole is greater than the width (w) of each shorter side of the slit-like recess (1b).

Description: BACKGROUND OF THE INVENTION: 1. Field of the Invention The present invention relates to an electron gun having an electrode structure improved in focusing characteristic and to a color cathode ray tube using such an electron gun. 2. Description of the Prior Art In general, in a color cathode ray tube such as a color cathode ray tube or a color monitor tube, to implement a so-called self-convergence function which causes a plurality of electron beams to converge on a phosphor screen without applying a special external correcting magnetic field on them, deflection magnetic fields for deflecting the electron beams emitted from an electron gun are given predetermined distortions. For this reason, the electron beams are subjected to deflection distortion while passing through the deflection magnetic fields, and form beam spots accompanied by haloes in the direction perpendicular to scanning lines, in the peripheral portion of a screen (phosphor screen), particularly in each corner portion of the screen. Thus, the focusing characteristic of the electron gun is degraded in such portions and, hence, the image quality is degraded. To attain the uniformity of the focusing characteristic over the entire screen, recesses each having a rectangular shape, i.e., so-called slits, are formed in a second grid electrode which constitutes the electron gun in such a manner that electron beam apertures are respectively located in the slit-shaped recesses. Figs. 1(a) and 1(b) are explanatory views showing the structure of the second grid electrode used in the above-described conventional electron gun. Fig. 1(a) is a front view viewed from the first grid electrode side, and Fig. 1(b) is a cross-sectional view taken along line A-A of Fig. 1(a). An electron beam aperture la of a second grid electrode 2 is a circular hole which pierces the center of a slit-shaped recess 1b formed and elongated horizontally in a plate which constitutes the second grid electrode. The diameter of the electron beam aperture 1a is smaller than the length of each shorter side of the slit-like recess 1b. The slit-like recess 1b is formed by coining in one plate which constitutes the second grid electrode, so that the slit-like recess 1b has a rectangular cross section which is open outwardly to a small extent from the bottom of the recess toward the open end, as shown in Fig. 1(b). Specifically, in the shown conventional electrode structure, a distance d1 exists between each longer side of the slit-like recess 1b and the edge of the electron beam aperture 1a, in the vertical direction, opened at the bottom of the slit-like recess 1b. As a result, the effect of suppressing haloes in the vertical direction which occur during scanning of the periphery of the screen is weakened. To solve the above-described problem, an electrode structure which will be described below has been proposed. Figs. 2(a) and 2(b) are explanatory views showing the structure of a second grid electrode used in another conventional electron gun. Fig. 2(a) is a front view of an electron beam aperture portion, and Fig. 2(b) is a cross-sectional view taken along line A-A of Fig. 2(a). This second grid electrode is made up of a first member 2-1 and a second member 2-2 which are fixedly bonded to each other, as by welding. The first member 2-1 has a slit 1b formed by press cutting, while the second member 2-2 has a circular electrode beam aperture 1a having a diameter of the same size as the length of each vertical (shorter) side of the slit 1b. By adopting such an arrangement, it is possible to reduce each of the distance d1 to zero, whereby it is possible to retain the aforesaid halo suppressing effect in the peripheral portions of the screen. To achieve a similar object, the electrode structure shown in Figs. 3(a) and 3(b) is disclosed in Japanese Patent Laid-Open No. 59637/1985 corresponding to Japanese Patent Application No. 164958/1983. Figs. 3(a) and 3(b) are explanatory views partially showing the structure of a second grid electrode used in still another conventional electron gun. Fig. 3(a) is a front view of an electron beam aperture portion, and Fig. 3(b) is a cross-sectional view taken along line A-A of Fig. 3(a). In this electrode structure, the diameter of the electrode beam aperture 1a is greater than each shorter side of the slit-like recess 1b and the vertical wall portions of the slit-like recess 1b are shared by the electron beam aperture 1a. Accordingly, it is possible to obtain an effect similar to that described above with reference to Figs. 2(a) and 2(b). SUMMARY OF THE INVENTION: However, any of the above-described prior art electrode structures still involve the following problems. In the case of the electrode structure shown in Figs. 2(a) and 2(b) formed by bonding the two members 2-1 and 2-2, during welding of both members 2-1 and 2-2, misalignment occurs between the slit 1b and the electron beam aperture 1a. Particularly if a vertical misalignment takes place, the effect of suppressing vertical haloes becomes unbalanced between the vertically upper and lower portions of the electron beam aperture 1a, and the focusing characteristic is deteriorated. This electrode structure also has a problem that the cost increases since the step of welding the two members 2-1 and 2-2 is needed. In the case of the electrode structure shown in Figs. 3(a) and 3(b), the vertical wall portions of the slit-like recess 1b which serves as a portion having a halo suppressing action are shared by the electron beam aperture 1a, with the result that the halo suppressing effect is marred. Specifically, as shown in Fig. 4, lines of electric force converge toward the electron beam aperture 1a, with the result that a vertical halo suppressing effect decreases and the focusing characteristic is deteriorated. The plate thickness of the plate member abruptly changes at the intersection (a portion B in Fig. 3(a)) of the slit-like recess 1b and the electron beam aperture 1a, and this abrupt change produces a burr which hinder a good electron beam aperture from being formed. An object of the present invention which has been made to solve the above-described various problems of the prior art is to provide an electron gun having an electrode structure in which slit-like recesses are respectively formed in electron beam aperture portions and decrease in a halo suppressing effect in a longitudinal (vertical) direction can be prevented, particularly in the peripheral portion of a screen, and to provide a color cathode ray tube equipped with such an electron gun. To achieve the above object, an electron gun according to the present invention at least comprises a triode part including a cathode, a first grid electrode and a second grid electrode, a pre-focusing part including the second grid electrode and a third grid electrode, and a main lens part including the third grid, ..., and an n-th grid electrode, in the order named. The electron beam aperture of at least one grid electrode of the first grid electrode and the second grid electrode is a rectangular hole and formed in the center of a slit-like recess having longer sides extending in one direction. The width of the rectangular hole in a direction perpendicular to the one direction is greater than the width of the slit-like recess in a direction perpendicular to the one direction. A color cathode ray tube according to the present invention comprises a vacuum envelope including a panel portion having phosphors formed on its internal face, a neck portion which accommodates an electron gun, and a funnel portion which connects the panel portion and the neck portion, and at least a shadow mask is suspended inside the vacuum envelope. In this color cathode ray tube, the electron gun at least comprises a triode part including a cathode, a first grid electrode and a second grid electrode, a pre-focusing part including the second grid electrode and a third grid electrode, and a main lens part including the third grid, ..., and an n-th grid electrode, in the order named. The electron beam aperture of at least one grid electrode of the first grid electrode and the second grid electrode is a rectangular hole and formed in the center of a slit-shaped recess having longer sides extending in one direction. The width of the rectangular hole in a direction perpendicular to the one direction is greater than the width of the slit-like recess in a direction perpendicular to the one direction. BRIEF DESCRIPTION OF THE DRAWINGS: Figs. 1(a) and 1(b) are explanatory views showing the structure of a second grid electrode used in a conventional electron gun, Fig. 1(a) being a front view and Fig. 1(b) being a cross-sectional view taken along line A-A of Fig. 1(a); Figs. 2(a) and 2(b) are explanatory views showing the structure of a second grid electrode used in another conventional electron gun, Fig. 2(a) being a front view of an electron beam aperture portion, and Fig. 2(b) being a cross-sectional view taken along line A-A of Fig. 2(a); Figs. 3(a) and 3(b) are explanatory views showing the structure of a second grid electrode used in still another conventional electron gun, Fig. 3(a) being a front view of an electron beam aperture portion and Fig. 3(b) being a cross-sectional view taken along line A-A of Fig. 3(a); Fig. 4 is an explanatory view of the distribution of lines of electric force at the second grid electrode which constitutes the conventional electron gun; Figs. 5(a) and 5(b) are explanatory views partially showing one embodiment of a second grid electrode which constitutes part of an electron gun according to the present invention, Fig. 5(a) being a front elevational view and Fig. 5(b) being a cross-sectional view taken along line A-A of Fig. 5(a); Fig. 6 is an explanatory view of lines of electric force at the second grid electrode which constitutes the electron gun according to the present invention; Figs. 7(a) and 7(b) are views of one transition step of a forming process, illustrating a method for manufacturing the second grid electrode according to the present invention, Fig. 7(a) being a plan view of the essential portion of the second grid electrode and Fig. 7(b) being a cross-sectional view taken along line A-A of Fig. 7(a); Figs. 8(a) and 8(b) are views of another transition step of the forming process, illustrating the method for manufacturing the second grid electrode according to the present invention, Fig. 8(a) being a plan view of the essential portion of the second grid electrode and Fig. 8(b) being a cross-sectional view taken along line A-A of Fig. 8(a); Figs. 9(a) and 9(b) are views of another transition step of the forming process, illustrating the method for manufacturing the second grid electrode according to the present invention, Fig. 9(a) being a plan view of the essential portion of the second grid electrode and Fig. 9(b) being a cross-sectional view taken along line A-A of Fig. 9(a); Figs. 10(a) and 10(b) are explanatory views showing one example in which the second grid electrode according to the present invention is used as a three-beam grid electrode, Fig. 10(a) being a front view and Fig. 10(b) being a cross-sectional view taken along line A-A of Fig. 10(a); Fig. 11 is an exterior view of an in-line type color electron gun to which the second grid electrode according to the present invention is applied; and Fig. 12 is a cross-sectional view showing a structure example of a color cathode ray tube using the in-line type color electron gun to which the second grid electrode according to the present invention is applied. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS: An electron beam aperture of the present invention has a rectangle shape having sides each of which is slightly greater than the width (perpendicular width, i.e., the length of each shorter side) of a slit-like recess. Accordingly, the effective portion of the slit-like recess has vertical walls which are equal in width to the electron beam aperture and which are parallel to each other. By adopting such a structure, parallel lines of electric force can be obtained, as shown in Fig. 6. Also, since the effective portion of a slit-like recess 1b and an electron beam aperture 1a are formed in common, it is possible to prevent positional misalignment of the electron beam aperture 1a and the slit-like recess 1b. Accordingly, it can prevent the unbalance in the effect of suppressing vertical haloes between the (vertically) upper and lower portions of the electron beam aperture 1a. Since the electron beam aperture 1a is a rectangular hole, it is possible to reduce the diameter of an electron beam spot without decreasing the luminance of the screen of the cathode ray tube, whereby the focusing characteristic can be improved. Further, since the wall of the slit-like recess 1b is inclined at an angle , it is possible to prevent an abrupt change in plate thickness from occurring at the intersection B (see Fig. 3(a)) of the electron beam aperture 1a and the slit-shaped recess 1b, whereby the burr occurrence during manufacture can be decreased so that an electron beam aperture of good shape can be obtained. [Embodiments] Preferred embodiments of the present invention will be described below with reference to the accompanying drawings. Figs. 5(a) and 5(b) are explanatory views of a first embodiment of an electron gun according to the present invention. Fig. 5(a) is a front view of the essential portion of a second grid electrode, and Fig. 5(b) is a cross-sectional view taken along line A-A of Fig. 5(a). In Figs. 5(a) and 5(b), reference numeral 1a denotes an electron beam aperture, reference numeral 1b denotes a slit-like recess, and reference numeral 2 denotes the second grid electrode. As shown in Fig. 5(a), the electrode beam aperture 1a is rectangular (square), and the length d of each side of the electron beam aperture 1a is 0.6 mm and the length w of each shorter side (width in vertical direction) of the slit-like recess 1b is 0.4 mm. In other words, the dimension d of each side of the electron beam aperture la is slightly greater than the length w of each shorter side of the slit-like recess 1b (d > w). It is preferable that the lengths d and w in mm. satisfy 0.5 ≥ d-W ≥ 0.2. In addition, the wall of the slit-like recess 1b is inclined at an angle  of 18°. It is preferable that this angle  of inclination be within a range of 10° to 30°. Incidentally, it is preferable that a depth u of the slit-like recess 1b be within a range of 50-80% of the height T of the inner wall of the electron beam aperture 1a. By adopting the above-described electrode structure, it is possible to produce parallel lines of electric force which perpendicularly act on an electron beam passing through the electron beam aperture 1a (see Fig. 6). Also, since the effective portion of the slit-like recess 1b and the electron beam aperture 1a are formed in common, it is possible to prevent positional misalignment between the electron beam aperture 1a and the slit-shaped recess 1b. Accordingly, it can prevent the unbalance in the effect of suppressing vertical haloes between the upper and lower portions (in vertical direction) of the electron beam aperture 1a. Since the electron beam aperture 1a is a rectangular hole, it is possible to reduce the diameter of an electron beam spot without decreasing the luminance of the screen of the cathode ray tube and thereby to improve the focusing characteristic. Further, since the wall of the slit-like recess 1b is inclined at the angle , it is possible to prevent an abrupt change in plate thickness at the intersection B (see Fig. 3(a)) of the electron beam aperture 1a and the slit-like recess 1b, whereby burr occurrences during manufacture can be decreased so that an electron beam aperture of good shape can be obtained. Figs. 7(a), 7(b) to 9(a), 9(b) are formation transition diagrams illustrating a method for manufacturing the second grid electrode according to the present invention, which has been described with reference to Figs.5(a) and 5(b). Figs. 7(a), 8(a) and 9(a) are plan views showing the essential portions of the second grid electrode, while Figs. 7(b), 8(b) and 9(b) are cross-sectional views taken along lines A-A of the respective figures. As shown in Figs. 7(a) and 7(b), a preliminary hole 4 having a diameter of approximately 0.5 mm is made in a plate 3 having a thickness of approximately 0.4 mm. Then, as shown in Figs. 8(a) and 8(b), the slit-like recess 1b is coined by using a lower machining jig (not shown) having a shape conforming to the shape of the slit-like recess 1b and an upper machining jig (not shown) having a flat shape. After that, as shown in Figs. 9(a) and 9(b), the electron beam aperture 1a is punched by using the lower forming jig having the shape conforming to the shape of the slit-like recess 1b and an upper forming jig (not shown) conforming to the electron beam aperture 1a. Finally, the plate 3 is punched into the desired exterior shape of the second grid electrode, thereby completing the second grid electrode. Figs. 10(a) and 10(b) are explanatory views showing an example in which the second grid electrode according to the present invention is formed as a grid electrode for an in-line three-electron-beam type electron gun. Fig. 10(a) is a front view, and Fig. 10(b) is a cross-sectional view taken along line A-A of Fig. 10(a). In this example, the slit-like recess 1b is so disposed as to face the first grid electrode. As shown in Figs. 10(a) and 10(b), an in-line type second grid electrode 5 is used in an in-line type color electron gun which includes three electrodes for green (G), blue (B) and red (R) in an integrated form. Although, in the above-described embodiment, the slit-like recess which extends in the horizontal direction is formed in the second grid electrode, the present invention is not limited to the aforesaid arrangement. For example, by applying the present invention to the first grid electrode, it is possible to provide a good result. Also, by extending the slit-like recess in the vertical direction, it is possible to provide a good result. Fig. 11 is an exterior view showing the appearance of an in-line type color electron gun in which the second grid electrode according to the present invention is used. A symbol K denotes a cathode, 10 a first grid electrode, 11 a second grid electrode, 12 a third grid electrode, 13 a fourth grid electrode, 14 a fifth grid electrode, 15 a sixth grid electrode, 16 a shield cup, 17 a bead glass and 18 a stem. As shown in Fig. 11, the cathode K, the first grid electrode 10 and the second grid electrode 11 constitute a triode part, and the second grid electrode 11 and the third grid electrode 12 constitute a pre-focusing part. The third to sixth grid electrodes 12-15 constitute a main lens, and these electrodes 12 to 15 are integrally fixed by the bead glass 17. Fig. 12 is a cross-sectional view showing an example of the structure of a color cathode ray tube using the in-line type color electron gun to which the second grid electrode according to the present invention is applied. A numeral 9 denotes an anode cap, 20 a face panel which constitutes a screen, 21 a phosphor screen, 22 a shadow mask, 30 a neck, 31 an electron gun, 40 a funnel, 41 an internal conductive film, 42 a magnetic shield and 50 a deflection yoke. The electron gun 31 shown in Fig. 12 is the electron gun described above with reference to Fig. 11. Electron beams emitted from the electron gun 31 are deflected in horizontal and vertical directions by the deflection yoke 50, then subjected to color selection by the shadow mask 22, and then projected onto predetermined phosphors which constitute the phosphor screen 21, thereby forming a color video image. As described above, in accordance with an example of the present invention, the size of a rectangular electron beam aperture is greater than the vertical width of a slit-like recess extending in the horizontal direction. Accordingly, it is possible to improve the effect of suppressing vertical haloes at the peripheral portion of a phosphor screen, so that the focusing characteristic in the peripheral portions of the screen can be made approximate to that at the center of the screen.

Claims: An electron gun (31) at least comprising in the order named as follows: a triode part including a cathode (K), a first grid electrode (10) and a second grid electrode (2,11); a pre-focusing part including said second grid electrode (2, 11) and a third grid electrode (12); and a main lens part including said third grid electrode (12) and n grid electrodes; characterized in that an electron beam aperture (1a) of at least one grid electrode of said first grid electrode (10) and said second grid electrode (2, 11) is a rectangular hole and formed in the center of a slit-like recess (1b) having longer sides extending in one direction; and the width of said rectangular hole in a direction perpendicular to said one direction is greater than the width of said slit-like recess (1b) in a direction perpendicular to said one direction. An electron gun according to claim 1, wherein said one direction is a horizontal direction. An electron gun according to claim 1, wherein said at least one grid electrode is said second grid electrode (2, 11). An electron gun according to claim 3, wherein said slit-like recess (1b) is formed in said second grid electrode (2, 11) on the side opposing said first grid electrode (10). A color cathode ray tube which comprises a vacuum envelope including a panel portion having phosphors formed on its internal face, a neck portion which accommodates an electron gun according to claim 1, and a funnel portion which connects said panel portion and said neck portion, and in which at least a shadow mask is suspended inside said vacuum envelope. A color cathode ray tube according to claim 5, wherein said one direction is a horizontal direction. A color cathode ray tube according to claim 5, wherein said at least one grid electrode is said second grid electrode (2, 11). A color cathode ray tube according to claim 7, wherein said slit-like recess (1b) is formed in said second grid electrode (2, 11) on the side which opposes said first grid electrode (10). A color cathode ray tube according to claim 6, wherein the following expression is satisfied: 0.5 ≥ d-w ≥ 0.2 where d represents said width in mm of said rectangular hole in the direction perpendicular to said one direction, and w represents said width in mm of said slit-like recess (1b) in the direction perpendicular to said one direction. A color cathode ray tube according to claim 5, wherein said slit-like recess (1b) includes a wall having an inclination with respect to a line perpendicular to a surface of said at least one grid electrode. A color cathode ray tube according to claim 10, wherein the angle of said inclination is within a range of 10° to 30°. A color cathode ray tube according to claim 5, wherein said n is 3. A color cathode ray tube according to claim 5, wherein the depth (u) of said slit-like recess (1b) is within a range of 50 - 80% of the height (T) of an inner wall of said electron beam aperture (1a).

Inventors: ENDO SATORU, IKEDA MAMORU, MISONO MASAYOSHI, SUGIYAMA MITSUHIRO, ENDO, SATORU, IKEDA, MAMORU, MISONO, MASAYOSHI, SUGIYAMA, MITSUHIRO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1994
Main classification: H01J29
IPC codes: H01J29